1. Field of the Invention
The present invention relates to cycle time calculating technology, and in particular to a computer-implemented method of calculating lot hold time.
2. Description of the Related Art
In an IC foundry, a lot refers to a batch of wafers. The cycle time of a lot processed by a working tool is decided by certain time factors, such as transport in time, Bank-Q time, process time, lot hold time, and transport out time. The transport in time represents the transfer time of the lot from a prior tool to the working tool. Bank-Q Time is time of the lot on the working tool waiting for processing. The process time is the actual working time on the working tool. The lot hold time indicates the time that the lot has to be held in the process, either single or multiple. The transport out time represents the transfer time of the lot from the working tool to the next tool.
Among the time factors, the lot hold time is the most uncertain. Thus, lot hold time creates a bottleneck in cycle time calculation of a lot. There are two main reasons for lot hold time uncertainty. One is a child lot problem and another is a multiple hold problem.
The child lot problem may occur after a lot is split. The conventional lot hold time calculating method only calculates the cycle time of the parent lot, ignoring the child lot. The multiple hold problem may occur after a lot processes multiple hold by different holders. The conventional lot hold time calculating method adopts the last lot hold to calculate the hold time, causing considerable inaccuracy.
U.S. Pat. No. 6,546,113 discloses a method of lot start that calculates virtual WIP time in a multi-product and multi-bottleneck manufacturing environment. The disclosed method is provided for calculating virtual WIP time (“VWIP”) in a multiple-bottleneck, multi-product manufacturing facility. The system and method provide calculation of one or more bottleneck VWIP values. Each of the bottleneck VWIP values represents the amount of work approaching one of n bottleneck workstations, where n>o. The work approaching the bottleneck workstation comprises at least one of m products, where m>o. The method, however, is not characterized by resolving the child lot and multiple lot hold problems of lot hold time calculation.